miller, I'm assuming that you work at MUAC so I'll bow to your greater knowledge of how things work but I had been led to believe that the legal basis for providing services - say, the Rules of the Air and differences filed to ICAO - is different over each territory.
While this may not have any effect on day-to-day operations, I can understand why some my consider that it is not a true FAB.
I've heard similar suggestions that, as a piece of airspace, it does not really 'enable optimum use of airspace, taking into account air traffic flows' but, rather, is an administrative arrangement which essentially results in a number of previously established sectors being brought under one service provider and management structure. Again, I can see their point.
I have no strong views myself. I just find it interesting that the EC introduced a concept that is defined in a very broad way, and supposedly the Commission is very keen to see the concept implemented sooner rather than later, quite possibly at significant cost (at least in Press Releases if the UK/IRL experience is anything to go by). And no-one really seems to know what it really is.
Taking my cynical hat off, I guess all successful things have got to start somewhere - and probably with something far less successful at the that time. We'll have to wait and see whether there are any real benefits to be had....